1. Field of the Invention
This invention relates to an aluminum alloy having excellent superplasticity.
2. Description of Prior Arts
A superplastic alloy generally means an alloy which exhibits an elongation at least 300% or higher when subjected to high temperature tensile deformation at a selected temperature normally in the range of from 400.degree. to 600.degree. C. and at a predetermined processing speed.
As in the deformation of plastics, superplastic alloy sheet, can, for example, be blow formed by air pressure to be integrally formed into products of complicated shapes or large sizes, with the result that superplastic alloys have been used as building material panels and aircraft parts in recent years.
At present aluminum, various kinds of superplastic alloys are known, one of which alloys is a JIS(AA) 5083 alloy.
The JIS(AA) 5083 alloy has a chemical composition consisting of 4.0 to 4.9% of Mg, 0.4 to 1.0% of Mn, 0.05 to 0.25% of Cr, wherein % means % by weight, and the balance of unavoidable impurities, has excellent corrosion resistance and anodic oxidation properties, is a non-heat-treating aluminum alloy having high strength, and exhibiting a high temperature elongation of about 300% as an evaluation standard of superplasticity at an initial strain rate of 1.1.times.10.sup.-3 / sec and at a deformation temperature of 540.degree. C.
Since the JIS(AA) 5083 alloy has such excellent properties, it has been used in various fields. However, superplasticity of which the elongation is about 300% is not satisfactory for meeting today's increased working demands.
Japanese Patent Publication No. 30392/1981 (priority, July 20, 1971, U.K. No. 33922; June 27, 1972, U.K. No. 33922), and Japanese Patent Laid-open Publication No. 161045/1980 (priority, July 20, 1971, U.K. 33992/71) disclose superplastic aluminum based alloys, which contain 0.3% to 0.8% of Zr. When Zr is added in an amount greater than 0.25%, the conventional casting process is liable to crystallize a giant compound of Zr with the result that steps of high temperature, and high cooling rate casting are required. Therefore, it is extremely difficult to produce large-sized direct chill (DC) casting slabs to provide problems in the production on an industrial scale.